Support complex types/values

[osandov]

The C standard defines complex types: https://en.cppreference.com/w/c/language/arithmetic_types#Complex_floating_types, as does the DWARF format. Before 78316a2, we had broken support for complex types, but now we don't have any code for them at all. To support them properly, we'd need to 1) bring back complex types and 2) implement objects with complex types.